It is hard to imagine a better role model for football than the star of a new
documentary about American Samoa - the world's worst international team

In a screening room in Soho last week, the former Chelsea and England defender Graeme le Saux stood up to introduce that rare thing: a genuinely important sports movie.

Next Goal Wins, a film created by the British duo Mike Brett and Steve Jamison, was originally supposed to be a standard David-v-Goliath tale about American Samoa, the world's worst international football team, as they attempted to qualify for this summer's World Cup.

As with many of the best documentaries, however, Brett and Jamison stumbled on something quite unexpected.

Yes, Next Goal Wins can still be seen as a Rocky-style triumph over adversity. But we have plenty of such movies already, both factual and fictive.

Where it really scores is as a parable of inclusiveness. Its undisputed star is Jaiyah Saelua – a member of Polynesia's fa'afafine or "third gender". She starts the film as a bit-part player, an exotic yet apparently insignificant squad member who admits: "I run like a girl." At first, her main contribution is to cheer on her more macho team-mates.

Yet something extraordinary happens when Thomas Rongen, a grizzled, chain-smoking Dutchman, is posted to Pago Pago by the United States Football Association as a troubleshooting coach.

Rongen had only taken the job as a way of exorcising a family tragedy: the loss of his 18-year-old daughter in a car crash. Once on the island, he forms an unlikely bond with Saelua. More than that, he sees something in her that all American Samoa's previous coaches had missed. Saelua had never even started a match before Rongen's arrival, but by the end of the film, she has become the defensive rock on which the team is built.

"When I first saw Jaiyah walk in, I nudged my wife in a cynical, sarcastic way and said: She must be the massage therapist,'" Rongen recalled, in an interview with The Telegraph. "When she said she was the centre-half, I swallowed a little but didn't say anything.

"What changed my mind is the way she got stuck in every day in training. In our World Cup qualifying game against Tonga, she put herself around and made one crucial clearance off the line. She has a keen understanding of positioning, which is what you want in a centre-half, along with mental and physical toughness. It became a no-brainer."

Saelua could never be mistaken for John Terry. She is 6ft but willowy and delicately built. She wears her fingernails long and paints her eyelashes, as is customary among the fa'afafine (which translates to "the way of the woman").

Celebrated within Polynesian culture, they are born as boys but brought up to be feminine, to dedicate themselves to the family, and sometimes to perform as dancers or singers.

The fa'afafine are not normally associated with sport, even if Manu Tuilagi's brother Olotuli, who prefers to be known as Julie, spent several years living in Leicester and attending the Tigers' matches.

But Saelua's role in rescuing the team that once lost 31-0 to Australia will surely change perceptions – and not only in the Pacific Islands.

How useful this film could be in breaking down the blinkered culture of professional football. Even by the standards of its peers, the sport suffers from alarmingly primitive values, as Le Saux – who put up with "gay" taunts throughout his career, mainly because he chose to read The Guardian rather than The Sun – is only too aware.

After the tournament, Saelua received a letter of congratulation from Fifa president Sepp Blatter, marking her status as the first transgender footballer to appear in a World Cup qualifying match (who keeps these statistics, anyway?).

Her herogram might seem ironic, given that Blatter is anything but progressive. The only memorable thing he has ever said about women in football is that they should wear tighter shorts.

But if Blatter really wants to reach a new audience, he should think about making Saelua a FIFA ambassador to the World Cup. It is hard to imagine a better role model for this stiflingly masculine game.